Will our military save us from alien invasions?

Definitely not. They will be completely incapable.

Knowledge of the speed of light, however, will save you.

Light speed cannot be exceeded in a vacuum no matter what. In fact, for aliens and Earthlings alike it would take thousands of years just to travel to our nearest star system. To travel from distant stars to ours would take thousands or millions of years, as it would take us to travel to them.

Fortunately, there will be no alien invasions. In fact, we may or may not know of any alien civilizations out there. So far we have discovered none.